The use of a wide variety of toxic chemicals in agriculture has presented recognizable problems, as well as potential hazards in terms of land use, and extant danger to wetland systems. Moreover, an array of toxic chemicals are discharged from diverse industrial facilities, these including accidental spills and the creation of dump areas which form a continuing source of pollution. These discharges, spills and dump sites create direct hazards to the use of land, and have caused serious drinking water contamination, and other health problems. Often these chemicals find their way into wetlands systems. For example, the coastal zone of Louisiana contains an ecosystem of more than seven million acres of marshes and estuaries representing approximately 40 percent of the total coastal wetland area of the lower forty-eight states of the United States. The gulfward movement of water introduces a wide variety of organic materials, including potentially hazard chemicals, into highly productive shrimp nursery grounds. The widespread use over the last several decades of herbicides, pesticides, and other chemicals in this geographical area has raised serious questions concerning the effect of these chemicals on the environment. Such toxic agents as, e.g., organo phosphates, organochlorides, polychlorobiphenyls (PCB's), polynuclear aromatic hydrocarbons (PAH's) and chlorinated phenols have been noted for their recalcitrant nature and the relative difficulty of establishing adapted microbial populations to effectively biotransform and biodegrade these materials within an acceptable time frame.
Large volumes of process waters, containing a changing organic chemical matrix of these and other toxic contaminants, are discharged by industrial plants located in this area following biological oxidation in aerated lagoons and/or activated sludge systems. Such aerobic systems have the ability to reduce the total BOD and COD of the effluents but are subject to upsets due to shifting effluent load from one toxicant class to another. It has been deemed advantageous to apply enzymes to the treatment of these systems since enzymes are biocatalytic materials which possess extraordinary high efficiency, have specific properties, and can be used to catalyze almost any chemical reaction, without producing harmful substances. Industrial applications of enzymes or microorganisms have been accomplished by using intact microorganisms or soluble enzyme preparations. It has been recognized that the immobilization of the catalyst offers a means of stabilizing the enzymes and microorganisms for subsequent recovery without inactivation. Whereas most immobilization studies are concerned with cell-free enzymes, more recently attention has been directed toward the use of immobilized whole microbial cells. Use of the whole microbial cell obviates the need for cell separation, enzyme extraction, and enzyme purification steps prior to immobilization.
Methods have been described in the literature for the immobilization of enzymes, microbial cells, plant cells and animal cells. In general, such methods have been classified as cross-linking, entraping and carrier-binding. The cross-linking method is based on the formation of chemical bonds, the immobilization of the enzymes or cells being accomplished by the formation of intermolecular cross-linkages between the enzyme molecules or the cells, by means of bifunctional or multi-functional reagents. Cross-linking reagents have included such compounds as glutaraldeyde, bisisoocyanate derivative and bisdiazobenzidine.
The entrapping method is based on a technique of confining the enzymes or cells in the lattice of a polymer matrix, or enclosing them in semipermeable membranes. The enzyme or cell itself is not physically bound to the gel matrix or membrane.
In the carrier binding method, the binding mode of which includes physical adsorption, ionic binding and covalent binding, the enzymes or cells are linked directly to water-insoluble carriers, e.g., polysaccharides (cellulose, dextron, and agarose derivatives), proteins (gelatin and albumin), synthetic polymers (ion-exchange resin and polyvinylchloride), and inorganic materials (brick, sand and glass). For the immobilization of cells the method has not been considered advantageous because leakage of the enzymes may readily occur due to autolysis during the enzyme reaction. In the covalent method of carrier-binding immobilization is carried out under more severe conditions than in the physical binding method, and accordingly conformational changes and partial destruction of the active center may occur. Often therefore immobilized enzymes having high activity are not obtained, or if obtained the activity decreases during long term operation and regeneration is not possible. With ionic binding immobilization of the enzymes can be achieved under relatively mild conditions, and accordingly relatively high activity is generally obtained. However, the binding forces between the enzyme and carrier are weak and changes in ionic strength, pH of the substrate or product solution can result in leakage in the enzyme from the mass.